An ultra-short ultra high-intensity pulse laser generator generates a linearly polarized pulse laser based on a mode locking technique and amplifies the generated pulse laser several times to obtain ultra high-intensity pulses. In this case, however, the linearly polarized pulse laser has an instantly high maximum output value, potentially damaging an optical device due to an electromagnetic field of the linearly polarized pulse laser.
In an effort to solve the problem, a Chirped Pulse Amplification (CPA) technique has been proposed. A laser generator based on the CPA technique, which includes an ultra-short, ultra high-intensity pulse laser oscillator, a pulse stretcher, amplifiers, and a pulse compressor, varies pulses of an ultra-short, ultra high-intensity pulse laser into temporally long pulses, amplifies the same, and subsequently returns them to the original pulse width. When the pulses of the ultra-short ultra high-intensity pulse laser are varied into temporally long pulses and amplified, the strength thereof is reduced, while the energy thereof is maintained as it is, and thus, the pulses can be amplified without damaging an amplifying medium.
Here, the laser generator based on the CPA basically uses linearly polarized light, due to the fact that the processes of generating, amplifying, stretching, compressing, and the like, of laser pulses in the laser generator, are generally designed to be optimized to use linearly polarized light.
A pulse stretcher and a pulse compressor respectively stretch and compress a pulse width by using a change in the speed of light according to a wavelength thereof, namely, a change in a length of a path (or an optical dispersion) using a change in a refractive index of light according to a wavelength of light generated when light passes through a medium. Also, in the case of correcting an error occurring as light passes through a medium, an increased pulsed width is corrected by varying a path of light according to a wavelength thereof. To this end, a mirror, a lens, a prism, a grating, and other elements, may be used, and here, such an optical device is also designed to use light linearly polarized in a particular direction.
Meanwhile, in order to convert linearly polarized light into circularly polarized light, a birefringent medium (which has birefringence and allows a speed of light to be varied according to an angle of linearly polarized light) is commonly used. In order to rotate a direction of linearly polarized light, a Faraday rotator is occasionally used by using magnetic characteristics of a medium.
When light passes through a pockels cell by using a birefringent medium according to an electrical signal applied from the outside, the pockels cell rotates to an angle of the linearly polarized light by 0°, 90°, 180°, or the like.
Crystal structures of the birefringent medium are not uniform according to an arrangement direction of the medium, and a refractive index of light may vary according to a wavelength thereof, depending on a direction in which light moves, a relationship with a polarizing angle, and an arrangement of a crystal structure. Further, a refractive index varies according to a relationship between an angle of linearly polarized light and a crystal structure. Namely, when light passes through such a medium, the speed thereof is varied, according to a direction of polarized light.
When linearly polarized light is intended to be converted into circularly polarized light, linearly polarized light is made incident at an angle of 45° with respect to a characteristic axis of the medium. The incident light is then disintegrated into two characteristically polarized light beams (characteristic polarization) within the medium, and thusly disintegrated linearly polarized light beams proceed at different speeds. The two linearly polarized light beams are combined at a rear surface of the medium upon passing through the medium, and at this time, when a phase difference of the two polarized light beams is 90°, they form circularly polarized light. When the phase difference of the circularly polarized light beams is +/−90°, an electric field is rotated horizontally relative to a movement axis, and this is known as left-circularly and right-circularly polarized light.
A birefringent optical device making a phase difference of linearly polarized light +/−90° is called a ¼ wave plate (or a quarter wave plate), and this plate may be easily damaged by an ultra-short, ultra high-intensity pulse laser, has severe optical dispersion, in particular, severe polarization dependent dispersion, so it is not easy to generate a circularly polarized pulse laser having an ultra-short ultra high-intensity pulse width. Of course, it is also not easy to convert linearly polarized light into an oval polarized light pulse laser.
Namely, according to the related art, polarization characteristics of a linearly polarized pulse laser cannot be controlled arbitrarily.